15.5 Systematic Inorganic Nomenclature: Ionic Compounds, Polyatomic Ions, Acids & Molecular Compounds

15.5 Systematic Inorganic Nomenclature: Ionic Compounds, Polyatomic Ions, Acids & Molecular Compounds

Quick Summary: The CLEP outline says descriptive chemistry should "develop an understanding of systematic nomenclature that facilitates communication." Nearly every question names substances, so a naming error can cost points even when the chemistry is understood. This section covers the naming rules for ionic compounds (including Stock-system Roman numerals), the common polyatomic ions, acids, hydrates, and binary molecular compounds. Coordination compounds are named in Section 9.3 and organic compounds in Section 16.2.

---

1. Monatomic Ions and Binary Ionic Compounds

• Cations keep the element name: $\text{Na}^+$ is sodium ion, $\text{Al}^{3+}$ is aluminum ion.
• Anions take the suffix -ide: chloride ($\text{Cl}^-$), oxide ($\text{O}^{2-}$), sulfide ($\text{S}^{2-}$), nitride ($\text{N}^{3-}$), phosphide ($\text{P}^{3-}$), hydride ($\text{H}^-$).
• Binary ionic compound: cation name + anion name, with no prefixes: $\text{MgCl}_2$ is magnesium chloride and $\text{Al}_2\text{O}_3$ is aluminum oxide. The charges already fix the formula, so prefixes are never used for ionic compounds.

Metals with More Than One Common Charge (Stock System)

Transition and post-transition metals that form more than one ion show the charge as a Roman numeral:

The older system uses -ous for the lower charge and -ic for the higher charge. Metals with only one common ion (Group 1, Group 2, Al, and usually $\text{Zn}^{2+}$ and $\text{Ag}^+$) do not take a Roman numeral. Mercury(I) is the diatomic ion $\text{Hg}_2^{2+}$, so its chloride is $\text{Hg}_2\text{Cl}_2$, not HgCl.

To find the Roman numeral, work backward from the anion charges: in $\text{Fe}_2(\text{SO}_4)_3$, three sulfates give $-6$, so each of the two irons is $+3$, and the name is iron(III) sulfate.

---

2. Common Polyatomic Ions

The Oxoanion Pattern

For a family of oxoanions of the same element, the endings and prefixes track the number of oxygens:

The same pattern names the bromine and iodine oxoanions (bromate $\text{BrO}_3^-$, periodate $\text{IO}_4^-$). The -ate ion is the reference form; -ite has one fewer oxygen with the same charge.

Writing Formulas from Names

Balance the total positive and negative charge with the smallest whole-number ratio, and put parentheses around a polyatomic ion that appears more than once: calcium phosphate is $\text{Ca}_3(\text{PO}_4)_2$ ($3 \times (+2) = +6$ and $2 \times (-3) = -6$), and ammonium sulfate is $(\text{NH}_4)_2\text{SO}_4$.

Hydrates

A hydrate's name gives the number of water molecules per formula unit with a Greek prefix: $\text{CuSO}_4 \cdot 5\,\text{H}_2\text{O}$ is copper(II) sulfate pentahydrate, and $\text{MgSO}_4 \cdot 7\,\text{H}_2\text{O}$ is magnesium sulfate heptahydrate.

---

3. Binary Molecular (Covalent) Compounds

Two nonmetals combine in more than one ratio, so the name must state the counts with Greek prefixes: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10).

1. Name the less electronegative element first (it is usually written first).
2. Give the second element an -ide ending.
3. Omit mono- on the first element, and drop the final a or o of a prefix before "oxide."

Some molecular compounds keep common names that you should recognize: water, ammonia ($\text{NH}_3$), methane ($\text{CH}_4$), hydrazine ($\text{N}_2\text{H}_4$), and nitrous oxide ($\text{N}_2\text{O}$, systematically dinitrogen monoxide).

---

4. Naming Acids

The hydro- prefix applies only to binary acids in water: pure $\text{HCl}(g)$ is hydrogen chloride, and it becomes hydrochloric acid when dissolved. A quick memory aid is "ate becomes -ic, ite becomes -ous."

---

5. Worked Examples

1. Name $\text{Fe}_2(\text{SO}_4)_3$: sulfate is $2-$; three sulfates total $-6$, so each Fe is $+3$. The name is iron(III) sulfate.
2. Write the formula for ammonium dichromate: $\text{NH}_4^+$ and $\text{Cr}_2\text{O}_7^{2-}$ combine 2 : 1, giving $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$.
3. Name KClO: $\text{ClO}^-$ has one fewer oxygen than chlorite, so it is hypochlorite. The name is potassium hypochlorite, the active ingredient of bleach.
4. Name $\text{HNO}_2(aq)$: it comes from nitrite, so the acid is nitrous acid.
5. Name $\text{N}_2\text{O}_5$: two nonmetals, so prefixes are needed. The name is dinitrogen pentoxide, the acid anhydride of nitric acid.
6. Spot the error in "copper oxide" for $\text{Cu}_2\text{O}$: copper forms $+1$ and $+2$ ions, so the Roman numeral is required. Two $\text{Cu}^+$ balance one $\text{O}^{2-}$, and the correct name is copper(I) oxide.